fix: cx-api can be installed twice by NPM

Fixes aws#17974

Inspired from aws/constructs#955

packages/@aws-cdk/aws-ecr-assets/test/image-asset.test.ts

@@ -452,7 +452,7 @@ testFutureBehavior('nested assemblies share assets: default synth edition', flag
 
   // Read the asset manifests to verify the file paths
   for (const stageName of ['Stage1', 'Stage2']) {
-    const manifestArtifact = assembly.getNestedAssembly(`assembly-${stageName}`).artifacts.filter(isAssetManifestArtifact)[0];
+    const manifestArtifact = assembly.getNestedAssembly(`assembly-${stageName}`).artifacts.filter(cxapi.AssetManifestArtifact.isAssetManifestArtifact)[0];
     const manifest = JSON.parse(fs.readFileSync(manifestArtifact.file, { encoding: 'utf-8' }));
 
     expect(manifest.dockerImages[DEMO_IMAGE_ASSET_HASH].source).toEqual({
@@ -464,7 +464,3 @@ testFutureBehavior('nested assemblies share assets: default synth edition', flag
 function isStackArtifact(x: any): x is cxapi.CloudFormationStackArtifact {
   return x instanceof cxapi.CloudFormationStackArtifact;
 }
-
-function isAssetManifestArtifact(x: any): x is cxapi.AssetManifestArtifact {
-  return x instanceof cxapi.AssetManifestArtifact;
-}

packages/@aws-cdk/aws-s3-assets/test/asset.test.ts

@@ -275,7 +275,7 @@ test('nested assemblies share assets: default synth edition', () => {
 
   // Read the asset manifests to verify the file paths
   for (const stageName of ['Stage1', 'Stage2']) {
-    const manifestArtifact = assembly.getNestedAssembly(`assembly-${stageName}`).artifacts.filter(isAssetManifestArtifact)[0];
+    const manifestArtifact = assembly.getNestedAssembly(`assembly-${stageName}`).artifacts.filter(cxapi.AssetManifestArtifact.isAssetManifestArtifact)[0];
     const manifest = JSON.parse(fs.readFileSync(manifestArtifact.file, { encoding: 'utf-8' }));
 
     expect(manifest.files[SAMPLE_ASSET_HASH].source).toEqual({
@@ -410,7 +410,3 @@ function mkdtempSync() {
 function isStackArtifact(x: any): x is cxapi.CloudFormationStackArtifact {
   return x instanceof cxapi.CloudFormationStackArtifact;
 }
-
-function isAssetManifestArtifact(x: any): x is cxapi.AssetManifestArtifact {
-  return x instanceof cxapi.AssetManifestArtifact;
-}

packages/@aws-cdk/cx-api/lib/artifacts/asset-manifest-artifact.ts

@@ -3,10 +3,33 @@ import * as cxschema from '@aws-cdk/cloud-assembly-schema';
 import { CloudArtifact } from '../cloud-artifact';
 import { CloudAssembly } from '../cloud-assembly';
 
+const ASSET_MANIFEST_ARTIFACT_SYM = Symbol.for('@aws-cdk/cx-api.AssetManifestArtifact');
+
 /**
  * Asset manifest is a description of a set of assets which need to be built and published
  */
 export class AssetManifestArtifact extends CloudArtifact {
+  /**
+   * Checks if `art` is an instance of this class.
+   *
+   * Use this method instead of `instanceof` to properly detect `AssetManifestArtifact`
+   * instances, even when the construct library is symlinked.
+   *
+   * Explanation: in JavaScript, multiple copies of the `cx-api` library on
+   * disk are seen as independent, completely different libraries. As a
+   * consequence, the class `AssetManifestArtifact` in each copy of the `cx-api` library
+   * is seen as a different class, and an instance of one class will not test as
+   * `instanceof` the other class. `npm install` will not create installations
+   * like this, but users may manually symlink construct libraries together or
+   * use a monorepo tool: in those cases, multiple copies of the `cx-api`
+   * library can be accidentally installed, and `instanceof` will behave
+   * unpredictably. It is safest to avoid using `instanceof`, and using
+   * this type-testing method instead.
+   */
+  public static isAssetManifestArtifact(art: any): art is AssetManifestArtifact {
+    return art && typeof art === 'object' && art[ASSET_MANIFEST_ARTIFACT_SYM];
+  }
+
   /**
    * The file name of the asset manifest
    */
@@ -36,3 +59,15 @@ export class AssetManifestArtifact extends CloudArtifact {
     this.bootstrapStackVersionSsmParameter = properties.bootstrapStackVersionSsmParameter;
   }
 }
+
+/**
+ * Mark all instances of 'AssetManifestArtifact'
+ *
+ * Why not put this in the constructor? Because this is a class property,
+ * not an instance property. It applies to all instances of the class.
+ */
+Object.defineProperty(AssetManifestArtifact.prototype, ASSET_MANIFEST_ARTIFACT_SYM, {
+  value: true,
+  enumerable: false,
+  writable: false,
+});

packages/aws-cdk/lib/api/cloudformation-deployments.ts

@@ -405,7 +405,7 @@ export class CloudFormationDeployments {
    */
   private async publishStackAssets(stack: cxapi.CloudFormationStackArtifact, toolkitInfo: ToolkitInfo) {
     const stackEnv = await this.sdkProvider.resolveEnvironment(stack.environment);
-    const assetArtifacts = stack.dependencies.filter(isAssetManifestArtifact);
+    const assetArtifacts = stack.dependencies.filter(cxapi.AssetManifestArtifact.isAssetManifestArtifact);
 
     for (const assetArtifact of assetArtifacts) {
       await this.validateBootstrapStackVersion(
@@ -437,7 +437,3 @@ export class CloudFormationDeployments {
     }
   }
 }
-
-function isAssetManifestArtifact(art: cxapi.CloudArtifact): art is cxapi.AssetManifestArtifact {
-  return art instanceof cxapi.AssetManifestArtifact;
-}